Chapter I. Basic Concepts

7/26/2019 Chapter I. Basic Concepts

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CHAPTER I

BASIC CONCEPTS1.1 Definitions1.2 Generating agents

1.3 Types of soils and characteristics

1.4 Structure of clay minerals

1.5 Structure of soils

Professors: A.Zepeda y E Rojas

January 17, 2011


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1.1 DEFINITIONS

Soil

Soils are formed with sediments and other solid particles that can be

separated in single particles with the force of your hand when

saturated regardless of whether or not they have content of organic

matter (Karl Terzaghi).

Sediments are produced by mechanical break down or chemicaldecomposition of rocks.

Soil Mechanics.

Soil mechanics is the application of the laws of mechanics andhydraulics to engineering problems dealing with soils.


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People use the land to live on, and build all sort of structures:

houses, roads, bridges, etcetera. It is the task of the geotechnical

engineer to predict the behavior of the soil as a result of these

human activities.

The problems that arise are, for instance, the settlement of a

road or a railway under the influence of its own weight and the

traffic load, the margin of safety of an earth retaining structure(a dike, a quay wall or a sheet pile wall), the earth pressure

acting upon a tunnel or a sluice, or the allowable loads and the

settlements of the foundation of a building.

For all these problems soil mechanics provides the basic

knowledge.


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Soil mechanics is the science of equilibrium and motion of soil bodies. Here

soil is understood to be the weathered material in the upper layers of the

earthscrust.

The non-weathered material in this crust is denoted as rock, and itsmechanics is the discipline of rock mechanics.

In general the difference between soil and rock is roughly that in soils it is

possible to dig a trench with simple tools such as a shovel or even by hand.

In rock this is impossible, it must first be splintered with heavy equipmentsuch as a chisel, a hammer or a mechanical drilling device.

The natural weathering process on a mass of rock produced by rain, ice

wind, gravity and temperature gradually reduce the rock in smaller

particles. This process starts by the fracturing of rock bodies during the

freezing and thawing of the water inside the fissures in the rock.


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n

t

n

Real soils have two types of structures; a

macrostructure consisting of large particlesand large pores and a microstructure

composed by packets of very small pores.

1.4 Air, water and solid phases


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WATER IN THE SOIL

Soil grain

Adsorbed water

Gas (Air)

Capillary

waterSaturated soil


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air

water

soil

solids

Weigth

Wa0

Ww

Ws

Wm

Volume

Va

Vw

Vs

Vm

Vv

TRADITIONAL THREE

PHASES DIAGRAM FOR

SATURATED SOIL


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1.2 GENERATING AGENTS

The Earths crust is mainly attacked by air , water and temperature. Soils

are formed by the weathering of rocks and minerals. The surface rocks

break down into smaller pieces through a process of weathering

All attacking mechanisms can be divided in two groups:Mechanical disintegration (mechanical weathering) and

Chemical decomposition (chemical weathering).


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Mechanical weathering : Refers to the weathering of the rocks by physical

agents such as periodic changes of temperature, the freezing of water in

the joints and cracks in the rocks, effects of living organisms such as plants

or animals.

Because of these phenomena rocks degrade into sands, in extreme cases

can produce silts and rarely clay.

Chemical weathering (decomposition): The action of agents who attack

the rocks by modifying its mineralogical or chemical structure.The principal agent is the water and the most important attack

mechanisms are oxidation, hydration and carbonation.

Chemical effects of vegetation also have influence on the process of

weathering. These mechanisms generate clay as final product.

The formation of soils has occurred through the geological eras.


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1.3 TYPES OF SOILS

Residual and Transported Soils

Residual soils: The material attacked by weathering agents

remains in the same place, directly above their parent rock.

There is a number of important characteristics of the parent

rock inherited by residual soils. These are the weathering profile

and the set of structures such as fissures, failure planes and

joints.


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Transported soils: These soils are removed from the parent

rock by the weathering agents. Different deposits of transportedsoil can be found in the same horizon without a direct

relationship between them.

Transported soils show a number of important characteristics

such as the stratigraphic profile and the thickness of eachstratum. We can have several strata without direct relationship

among them.


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A Horizonor vertical washing area: is the mostsuperficial and it has roots and vegetation. Its

color is usually dark due to the abundance of

decomposed organic matter or humus,

determining the passage of water that drag

downward fragments of fine size and soluble

compounds. Highly leached materials (Lixiviation

or eluviation zone).

B Horizon or precipitation area: it lacks of

organic material, so its color is lighter, In the B

horizon materials drawn from above are

deposited, mainly clay materials, oxides and

metal hydroxides, carbonates, etc. (eIuviation

zone).C Horizon or parent rock: is made up to top of in

situ rocky material. Soil rests on horizon C, more

or less fragmented by mechanical and chemical

action, but it still recognizes it's original

characteristics.

Horizons of Soil.


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Transported Soils

There is a number of transport agents in nature, the

main ones are:

Glaciers

Wind

Rivers

FloodingsSeas

Gravity

These agents frequently work combined.


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Types of soil deposits

a)Glacial deposits

b) Moraine or Gravity deposits

c) Alluvial deposits

d) Fluvial deposits

e) Lacustrine deposits

f) Eolian deposits

g) Marine deposits

h) Organic Soil


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Western slopes of theSouthern Alps, South

Island, New Zealand

(G.R. Roberts, Nelson,

New Zealand).

Glacial deposits: The glaciers, in

their movement, generate big

pressures and abrasive effect on

rocks. The eroding action of the

glacial ice crushes and pulverizesthe parent bedrock and transforms it

into silt, sand, and gravel. This could

be accomplished due to the great

depths and enormous pressure of

glacier ice. There are several kinds

of glacial deposits, the main aremoraines.


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Moraine deposits
http://en.wikipedia.org/wiki/File:Moraines_Surlej.jpg


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A moraine


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The most recent glaciation on the North American continent

is the Laurentide (or Wisconsian) glaciation, which

dissappeared some 10 000 to 13 000 years ago.


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Fluvioglacial soil

Glacial deposits in the

Bicentenario Bridge,

Quertaro


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Gravity deposits: Moraine deposits, are sediments that accumulate at

the foot of mountain slopes because of avalanches, slides or instability of

the material on the slopes.


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Alluvium deposits: Sediment deposited by flowing water as in a flood plain

or delta. It is also called alluvium. Their grain size varies from large rock

fragments, gravel, sand, silt and some clay.

The term "alluvium" is not typically used in situations where the formation of

the sediment can clearly be attributed to another geologic process that is

well described. This includes (but is not limited to): lake sediments

(lacustrine), river sediments (fluvial), or glacially-derived sediments (glacial

till).

Alluvial plaine in Red Rock Canyon Park
http://en.wikipedia.org/wiki/File:AlluvialPlain.JPG


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Alluvial Soil

Alluvial soil deposit in Quertaro

El t i i f


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Slickensides, the glossy (shiny and smooth) surfaces seen in

this picture are an obligatory requirement for vertisols

(USDA).

Electronic microscopy of sweep


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Picture of a soil layer in Bulgaria. There are carbonate

concretions.


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Fluvial Soil in a Delta

River deposits (fluvial): Coarse particles are graved dawn stream during

flooding and deposited when a decrease in the water velocity occurs. Fine

particles remain in suspension to be deposited in quieter waters. Thus, river

deposits are segregated according to size.


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The coarse stones that are created close to the mountains are

transported downstream by gravity water flow.

The stones are gradually reduced in size, so that the material becomes

finer: gravel, sand and eventually silt.

The material is deposited by flooding or flowing rivers. The coarsest

material at high velocities, but the finer material only at very small

velocities. This means that gravel will be found in the upper reaches ofa river bed, and finer material such as sand and silt in the lower

reaches (Arnold Verruijt, Delft University of Technology, 2001).


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Lacustrine material

Lacustrine deposits: Fine and very fine sediments like silts and clays are

deposited when running water comes to rest, like in lakes or deltas


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Wind deposit: Sand dunes in the Sahara, Morocco.

Wind deposits (Aeolin or Eolian): Loess and dune sands. This

sediments are typical of arid regions, and the water table is encountered at

great depth from the ground surface.


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Organic material

Marsh in Coatazacoalcos Ver.


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Marine soil

Marine deposits: They are related to the characteristics of the

rock formations eroded by sea water


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COARSE GRAINED SOIL MINERALS

Silicates of aluminium: Feldespar (of potassium,

sodium or calcium), micas, olivine and serpentine.

Oxides: Quartz, limonite, magnetite and corundum.

Carbonates: Calcite and dolomite.

Sulphates: Anhidrite and gypsum


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1.4 STRUCTURE OF CLAY MINERALS

Clay minerals are predominately hydrated silicates of aluminum and/or

iron and magnesium or other metals. These minerals are

predominantly crystalline in that the atoms composing them are

arranged in definite geometrical patterns.

Most of the clays minerals have sheet or layered structures.

Soil masses generally contain a mixture of several clay minerals.

Clay minerals are very small (less than 0.002 mm) and very

electrochemically active particles which can be seen using an electron

microscope.

There are two fundamental units (building blocks) for the clay mineral

structure:

Silica unit (Tetrahedron unit)

Aluminum unit (Octahedral unit)


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Oxygen

Silicon

Silica tetrahedron unit,

approximately 4.6 Ahigh

Block equivalent=

Angstrom unit A= 10-10m


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AluminumOxygen

Octahedral unit,

about 5.05 Ahigh

Octahedron

= Block equivalentG or B

G= Gibbsite (Al)

B= Brucite (Mg)


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According to the mineral structure of clay, some of the most common clay

minerals are the following

Kaolinite: Its structure is very stable in the presence of water.

Montmorillonite: Its structure is very unstable in the presence of

water. When the water content increases it swells.

Illite: Its behavior is intermediate between kaolinite and

montomorillonite

Kaolin is used in ceramics, medicine, coated paper, as a food additive,

in toothpaste, as a light diffusing material in white incandescent light

bulbs, and in cosmetics. It is generally the main component in porcelain.

Kaolin clay is widely used as an intestinal absorbent to combat intestinalinfections, in antidiarrheal medicines and for digestive disorders.

Kaolinite and halloysite clays are widely used for chinaware and

ceramic, due to absence of iron and subsequent iron decoloration at

high tempertures.
http://en.wikipedia.org/wiki/Light_bulbshttp://en.wikipedia.org/wiki/Light_bulbs


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7 ASi

Al

Kaolinite,

schema of the

basic unit

Al

Si

Si

Schema of the

Montmorillonite

9.6 A

Schema of the Illite

Al

Si

Si

10 A

K, Potassium



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> 9.6

Cationes de cambio n H2O

Oxgeno

Hidroxilo, OH

Al, Fe, Mg

Si, a veces Al

Chemical structure of the montmorillonite

Oxygen

Silicon

Aluminium



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agua capilar

agua

adsorbida

The phenomenon of capillarity in addition to the ionsin adsorbed water layer produce suction on

unsaturated soils (matric and osmotic suction).

Adsorbed

water

Capillary water

Clay particles

Negative electrical forces on the

surface of the mineral particles

attract water molecules and ions.

These forces decrease as the water

molecules and ions are farther

away from the surface of the grain.


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Floculated and dipersive clays depend on their mineralogy

and the environmental conditions in which they were formed

Distance

Attractio

n

Rep

ulsion

Floculated clay

Dispersive clay


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The specific surface is the sum of all the external area ofthe particles of soil per gram of material. A fine sand has

around 0.04 m2 of specific surface. A caolinite has 10 m2,

ilite has 100 m2 and montmorillonite has up to 800 m2.

The atoms of oxigen of the alluminate sheets are located atthe top and bottom of the clay particles in the illite and

montmorillonite. This produces a negative charge on the

flat surface of the particles.

Plasticity is the ability of soils to absorb water and todeform without fissuring. Clean sands have no plasticity

and montmorilonites are the most plastic of soils.

Specific surface and plasticity


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Water is strongly attracted by minerals. In the case of clays

plasticity is generated by the viscous water layer.

Coarse materials have much smaller specific surface andless affinity to water (the viscous layer is almost non

existent) and therefore do not develop significant

plasticity.


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Visc

osity

Distance from the particle surface, d

IIII II

-

-

-

-

-

-

-

-

-

-

-

-

-

--

+

+

Solid particle

Solid water layer (I)

Viscous water layer (II)

d

I

II

Free water (III)

The electromagnetic forces reduce as the molecules of water move away from the

surface of the clay particle

particle


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Cation Exchange Capacity (CEC)

Cations are positively charged ions.

Cation exchange capacity (CEC) represents the quantity of

negative charges existing on the surfaces of clay particles. The

negative charges attract the cations, hence the name cationexchange capacity.


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Diagram of a particle of clay with negative charges

on the surface attracting different cations.

Different types of clays have different CECs


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The most common soil cations, (including their chemical symbol and charge)

are: calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+),

hydrogen (H+), sodium (Na+), aluminum (Al+++), iron (Fe++), zinc (Zn++) and

copper (Cu++). Notice that some cations have more than one positive charge.

The capacity of the soil to hold on these cations is called the cation exchangecapacity (CEC).

The quantity of positively charged ions (cations) that a clay mineral or similar

material can accommodate on its negatively charged surface is expressed as

milliion equivalent per 100 g, or more commonly as milliequivalent (meq) per

100 g or cmol/kg.(10 cmolc/kg = 10 meq/100 g).

Clays are aluminosilicates in which some of the aluminum and silica ions

have been replaced by elements with different charge. For example,

aluminum (Al3+) may be replaced by iron (Fe2+) or magnesium (Mg2+), leading

to a net negative charge.


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The CEC of the soil will depend on the pH of the soil. A neutral soil (pH ~7)

will have a higher CEC than a soil with for example pH 5. Or in other words,the CEC of a soil with pH-dependent charge will increase with an increase in

pH.

This principle applies to increasing pH by the presence of lime for the

stabilization of expansive clays.

Expansive clayis a soil that is susceptible to large volume changes directlyrelated to changes in water content. Mitigation of the effects of expansive

clay on structures remains a major challenge in geotechnical engineering.


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Clay Exchange capacity, meq/100gr

Kaolinite 3-15

Halloysite 10-40

Illite 10-40

Vermiculite 100-150

Montmorillonite 80-150


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Identification of Clay Minerals

X Ray Diffraction: Each diffracted path is the image of different atomic

planes.

Electron microscope.

Differential Thermal Analysis (DTA): consists of simultaneously heating a

test sample and a thermally inert substance at constant rate (usually about

10C/min) to over 1000C and contiunuously measuring differences in

temperature between the sample and the inert material.

Chemical Analysis

Thermogravimetric Analysis


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x ray diffractometer . Equipped for qualitative and quantitative analysis of

crystalline compounds


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x ray difraction results

Difraction angle

Frequency

Al Mg Si


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Thermal analysis Temperature range is -100 to 1600C for simultaneous

DSC/DTA/TG, room temperature to 1600C for dilatometry


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TD

TD

Exothermic

Reaction

Endothermic

Reaction

Typical thermogram for soil minerals

a

a is the amplitude (cm);Ais the area (cm2)

A

1000C0


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Particle diameter (mm)

Gravel 4.75 76.2

Sand 0.075 4.75

Silt 0.005 0.075Clay 0.002 - 0.005

Colloid < 0.002Flat/Needle

Particles

Granular

Particles


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Shape of Granular Soil Particles

Angular: Short transported distance

Round: Large transported distance

ifi i f i i l


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Specific gravity of important minerals


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a) Soil Structure of coarse particles (simple structure): mainly

gravity forces are involved.

Assemblage of individual particles

Bulky particles

Dense Bulky SoilLoose Bulky Soil

Voids (Vv)

Bulky particle (Vs)

e=0.91 e=0.35


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Granular Loose Soil

Honeycomb soil structure

in granular soil

Loess (wind) deposit

Good compressive

Strength

Unstable if loaded

(collapse)


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Most soils contain a variety of particle sizes

Granular Soils Packing

In general, the greater the range of the size of soil

particles, the lower the void ratio because small particles

fill the voids of larger particles

Soil strength also depends on particle interlockGreater interlock higher shear strength

Interlock is a function of: particle shape and

amount of inter-particle contact


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Granular Soil Volumetric weigth

Very looseLoose

Mediun compact

Dense or compact

Very dense or very compact

High void ratio (e)Low strength, compressible

Low void ratio (e)

High strength, incompressible

Granular Dense Soil

Dense soil:

Low Vv, high Vs, therefore low e

Lower the soil void ratio the more dense,

Less settlement and higher strength


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minmax

max

ee

eeDr n

A measure of the structure of coarse soils is given by

the relative density parameter , Dr

minmax ,ee Are the laboratory values of themaximun and minimum void reatios

s

v

n

V

Ve Is the natural value of void reatio

The densest state, ,is obtained by vibrating a

confined weight of sand and measuring the volume.

The loosest state, is obtained by carefully placing

the material in a mould.

mine

maxe

Vv= volume of voids, Vs= volume of solids


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Flat clay particles

Flat thin particles of clays and colloids

cohesive soils

Large surface area with small mass

Specific surface = surface area/mass

Electrical forces dominate their behavior

Negligible gravity forces

Sand = 0.001 to 0.4 m2/gram

Silt = 0.4 to 1.0 m2/gram

Clay = 5 to 800 m2

/gram


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Dispersed

structure

d) Dispersed - particles repel each other

(negative to negative charge).

c) Flocculated clay or cardhouse structure: particles attract to each


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Plate particles: Flaky or mica shape

Edge to face contact. It is

called flocculated or

cardhouse strcture

Due to electrical forces and deposition in very low energy

environments, clays develop structures with large voids ratio

) y p

other in lakes or quiet rivers in edge to face contacts then settle

dawn due to gravity

Clay Structure


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a) Honeycomb structure: particles deposit due

to gravity forces. During this process they areattracted and attached to others

Clay Structure(particles equal or smaller than 0.002 mm in size).


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Floculated structure or superior order honeycomb


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b) Composite structure: Clay particles are aggregated or flocculated

together in submicroscopc fabrics units which are called domains. Domains

form groups called clusters and clusters form peds and group of peds of

macroscopic size.


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Composite structure


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Typical Kaolinite


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Electronic microscopy of sweep, natural soil

of Queretaro (Lpez-Lara, 2001).

Chemical analysis, Tejeda clay


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2.13Non soluble

material

0.06K2O

0.72Na2O

5.36MgO

66.52CaO

21.75%Lost by

calcination

3.663.94Potassium

1.791.79Sodium

35.6733.95Calcium

8.538.48Magnesium

Interchangeable cations

(meq/100g)

7.78.2Ph

48.86

me/100g

49.96

me/100g

Cation Exchange capacity

(CEC)

1.631.73Organic material

0.400.47MgO

1.131.36K2O

3.634.34CaO

6.716.74Fe2O3

19.9319.58AL2O3

55.76%54.13%SiO

21

SampleSubstance

Lime used in the

stabilization


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MONTMORILLONITE

2


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SILTSIZE FRACTION OF MEXICO CITY CLAY SHOWING FRAGMENTS OF SHELLS

(DIATOMS). (DIATOMSARE A MAJOR GROUP OF ALGAE MOST DIATOMSARE

UNICELLULAR, ALTHOUGH THEY CAN EXIST AS COLONIES).

20

Needle particles


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Halloysite nanotubes imaged at

Cornell University

A bundle of NaturalNanohalloysite nanotubes

compared to the width

of a human hair

Sensitivity and Thixotropic


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y p

Properties of Clays

Sensitivity, St: The ratio of peak undisturbedstrength to remolded

strength, as determined by unconfined compression test, was used

initially as a quantitative measure of sensitivity (Terzaghi, 1944).

St = qu/ qr

Thixotropy: Is defined as an isothermal, reversible, time- dependentprocess occurring in a material under conditions of constant volume

whereby it stiffens while at rest and softens or liquefies upon

remolding.

The remoulding of thixotropic clays causes an abrupt drop in pore

water tension (increase in pore water pressure) generating an

important reduction of its strength followed by a slow regain instrength during periods at rest.


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Thixotropic properties of clays

Strength

Time

Agedstrength

Remolded

Remolded

Remolded Strength

Sa

Sr

Sa/Sr = Thixotropic Strength Ratio


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Extremely sensitive clays losse

strength and become fluid when

remoulded. They are called quick

clays. (Photograph courtesy of

Haley and Aldrich, Inc.)


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St

Insensitive ~ 1.0

Slightly sensitive clays 1-2

Medium sensitive

clays

2-4

Very sensitive clays 4-8

Slightly quick clays 8-16

Medium quick clays 16-32

Very quick clays 32-64Extra quick clays >64

Classification of Sensitivity Values


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CAUSES OF SENSITYVITY

At least six differente phenomena may

contribuite to the develpment of sensitivity:

1. Metastable fabric;2. Cementation;

3. Weathering;

4. Thixotropic hardening;

5. Leaching, ion exchange;

6. Formation or addition of dispersing

agents.


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Honeycomb structure


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Colloidal particles: Grain size less than 0.0002 mm:If the colloidal particles are in a water suspension, there is

no effect of the gravity forces, they are in Brownian

movement, it is no possible the sedimentation. There is

repulsion force among the particles, caused by the negative

electrical charge on the surface of the grains.

Only with the addition of a electrolyte to the suspension for

neutralize the negative charge is possible the fluccullation

of particles and then the deposition


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Apparatus

Apparatus for DTA consists of a:

Sampler holder, usually ceramic, nickel, or platinum.

Furnace.

Temperature controller to provide a constant rate of heating.

Thermocouples for measurements of temperature and thedifference in temperature between the sample and the inert

reference material.

The recorder for the thermocouple output.

The amount of sample required is about 1 gr.


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Typical Kaolinite


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Element Weight% Atomic%

O K 14.83 26.83Na K 0.75 0.94

Mg K 2.25 2.68

Al K 2.75 2.95

Si K 46.61 48.04

K K 2.31 1.71

Ca K 5.01 3.62

Fe K 25.50 13.22

Totals 100.00

Spectrum processing:

Peaks possibly omitted: 2.149, 8.036, 8.475, 8.900, 9.706, 11.501, 13.367 keV

Processing option: All elements analyzed (Normalised)

Number of iterations = 2. Jurica Clay, Queretaro city. Mexico. Alicia Del Real L

JU100-2 sitio 1 espectro2

1 3 Characteristics and structure of soils


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1.3 Characteristics and structure of soilsMercury porosimeter measures pore size distribution in the range of pore diameters

between 300m and 3 nm. Samples can be in the form of granular powders or monoliths.

Nitrogen adsorption the pore size distributions in the range of pore diameters between 100

nm and 1.7 nm, the specific surface area, micropore volume and area, and total porevolume.

Mercury intrusion

Nitrogen adsorption


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O K 18.71 31.06

Na K 0.68 0.79

Mg K 1.75 1.92

Al K 10.36 10.20

Si K 46.95 44.40

K K 2.59 1.76

Ca K 4.57 3.03

Fe K 14.38 6.84

Totals 100.00

Spectrum processing :

Peaks possibly omitted : 2.149, 8.036, 8.515, 8.905, 9.706, 11.486, 13.350 keV

Processing option : All elements analyzed (Normalised)

Number of iterations = 3. Jurica Clay, Queretaro city. Mexico. Alicia Del Real L.

Sample: JU100 2 espectro4. Site of Interest 2 Type: Default. JU100-2 sitio 2


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O K 18.71 31.06

Na K 0.68 0.79

Mg K 1.75 1.92

Al K 10.36 10.20

Si K 46.95 44.40

K K 2.59 1.76

Ca K 4.57 3.03

Fe K 14.38 6.84

Totals 100.00

Spectrum processing :

Peaks possibly omitted : 2.149, 8.036, 8.515, 8.905, 9.706, 11.486, 13.350 keV

Processing option : All elements analyzed (Normalised)

Number of iterations = 3. Jurica Clay, Queretaro city. Mexico. Alicia Del Real L.

Sample: JU100 2 espectro4. Site of Interest 2 Type: Default. JU100-2 sitio 2

BENTONITE


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BENTONITE

Bentonite usually forms from weathering of volcanic ash, most often in the

presence of water. For industrial purposes, two main classes of bentonite exist:

sodium and calcium bentonite.

Sodium bentonite expands when wet, possibly absorbing several times its dry

mass in water. Because of its excellent properties it is often used in drilling mud

for oil and gas wells and for geotechnical and environmental investigations.

The property of swelling also makes sodium bentonite useful as a sealant,especially for the sealing of subsurface disposal systems for spent nuclear fuel

and for quarantining metal pollutants of groundwater. Similar uses include

making slurry walls, waterproofing of below-grade walls and forming other

impermeable barriers: e.g., to seal off the annulus of a water well, to plug old

wells, or as a liner in the base of landfills to prevent migration of leachate.

Sodium bentonite can also be "sandwiched" between synthetic materials tocreate geo-synthetic clay liners (GLC) for the aforementioned purposes. This

technique allows for more convenient transport and installation and it greatly

reduces the volume of sodium bentonite required.


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A slurry wallis a technique used to build reinforced-concrete walls

in areas of soft earth close to open water or with a high ground

water table.

A trench is excavated to create a form for each wall. The trench is

kept full of slurry at all times. The slurry prevents the trench from

collapsing by providing outward pressure which balances the

inward hydraulic forces and prevents water flow into the trench.Reinforcement is then lowered in and the trench is filled with

concrete, which displaces the slurry.

Slurry walls are typically constructed by starting with a set of guide

walls, typically 1 meter deep and 0.8 meter thick. The guide wallsare constructed on the ground surface to outline the desired slurry

trench and guide excavation.
http://upload.wikimedia.org/wikipedia/commons/2/29/SlurrywallEquipment.jpg


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Slurry wall excavator

Excavation is done using a special clamshell-

shaped digger or a hydromill trench cutter.

The excavator digs down to the required

depth, or bedrock, for the first cut. Theexcavator is then lifted and moved along the

trench guide walls to continue the trench with

successive cuts as needed. The trench is kept

filled with slurry (usually a mixture of bentonite

and water) at all times to prevent collapse.

Once a particular length is reached, a

reinforcing cage is lowered into the slurry-

filled pit and then the pit is filled with concrete

from the bottom up using tremie pipes. The

concrete displaces the bentonite slurry, whichis pumped out and recycled.
http://upload.wikimedia.org/wikipedia/commons/2/29/SlurrywallEquipment.jpg


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Schema of the process of building a Milan Wall: Excavation,

placement of joints between panels, placement of steel

reinforcement, filled with concrete from the bottom up using

tremie pipe.

Characteristics:


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Steel reinforcement for Milan Wall 27 m

deep, Metro, line B.Extraction of metalic joints.

Metro . Mexico City.

Characteristics:

Width: 60 cm (standard) 80 cm

Depth: up to 45 m

Mud: Bentontic polymer


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Completed Milan Walls.

Metro, Mexico City


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Chapter I. Basic Concepts

Documents

Transcript of Chapter I. Basic Concepts